Leviticus 13:3

and the priest shall look on the plague, on the spot or sore, in the skin of the flesh; and when the hair in the plague, on the suspicious spot, is turned white, this sign was all the more striking since the hair of the Jews was normally black, and the plague in sight be deeper than the skin of his flesh, when the spot appeared to have sunken into the skin, it is a plague of leprosy; and the priest shall look on him and pronounce him unclean. The name leprosy is derived from a word meaning "to strike, to strike to the ground," the leper being a person who has been stricken or smitten by God. The sickness is contagious only, but was and is commonly treated as being infectious as well. Three forms of the disease were distinguished in olden times: the white leprosy, which was very prevalent among the Hebrews, the tubercular leprosy, or the Egyptian boil, and the black leprosy.